The widespread availability of low-cost portable computer equipment has meant that more and more people use computers away from a fixed location. With the equally prolific availability of low-cost cellular telephones, sophisticated portable computer users now demand the same type of data transmission services, using the cellular telephone network, which are presently widely available using the landline public-access switched telephone network (PSTN).
Unfortunately, it has not been generally possible to directly connect the most popular type of high-speed modems to standard voice-grade cellular telephone lines. In particular, certain specifications such as the V.22bis and V.42bis standards of the Consultative Committee on International Telegraph and Telephone (CCITT) permit high-speed data transfer at 2400 and 9600 bits per second. These CCITT-compatible modems are gaining widespread acceptance among the computing community in both North America and Europe, since they do not use more than 3 kilohertz (kHz) or so of bandwidth, and thus do not require special telephone land lines. This means that a fixed-location computer user may simply purchase a CCITT-compatible modem, connect it to a standard telephone land line, and begin to transmit and receive data at high speeds, without first requesting a special type of line from the local telephone company.
However, CCITT-compatible modems cannot presently be reliably connected to standard voice-grade cellular telephone equipment. When such an attempt is made, severe and unpredictable distortion and concomitant loss in signal quality may occur, because the signalling requirements of present cellular systems are incompatible with CCITT signalling requirements.
As such, in order to reliably provide CCITT-compatible modem signalling over a cellular network, cellular modems are presently sold in matched pairs. One modem serves as a cellular subscriber unit and is connected to the portable computer; the second modem must be used to connect the fixed-location computer to the land-based PSTN. The matched second modem serves to recondition the CCITT-format signals received from the fixed-location computer to insure compatibility with cellular signalling requirements.
Thus, in order to reliably connect a CCITT-compatible modem to the cellular network, arrangements must be made in advance to insure that the computer equipment at both ends have a cellular-compatible modem. This, in turn, not only complicates the transmission of data over the cellular network, but also limits the user of the portable computer, who can only communicate with other computers that have cellular-compatible modems.
What is needed is a way to allow a cellular subscriber modem to be connected to the cellular network as easily as a land-based modem can be connected to the PSTN, that is, without requiring prior intervention to insure that the land-based modem on the other side of the PSTN is cellular-compatible.
As a result, by eliminating the need to connect cellular modems in pairs, a cellular subscriber modem could then be treated as any other type of modem. Assuming arrangements have been made for cellular voice service, the user could simply connect such a cellular modem connected to a portable computer and then begin to reliably transmit and receive data at high speeds.
The implementation of such a cellular modem should avoid the need for specialized hardware as much as possible, so that existing cellular telephone and modem integrated circuit (IC) chips may be used.